Extracellular vesicles in the tumor microenvironment: Therapeutic resistance, clinical biomarkers, and targeting strategies

Numerous studies have proved that cell-nonautonomous regulation of neoplastic cells is a distinctive and essential characteristic of tumorigenesis. Two way communications between the tumor and the stroma, or within the tumor significantly influence disease progression and modify treatment responses. In the tumor microenvironment (TME), malignant cells utilize paracrine signaling initiated by adjacent stromal cells to acquire resistance against multiple types of anticancer therapies, wherein extracellular vesicles (EVs) substantially promote such events. EVs are nanoscaled particles enclosed by phospholipid bilayers, and can mediate intercellular communications between cancerous cells and the adjacent microenvironment to accelerate pathological proceeding. Here we review the most recent studies of EV biology and focus on key cell lineages of the TME and their EV cargoes that are biologically active and responsible for cancer resistance, including proteins, RNAs, and other potentially essential components. Since EVs are emerging as novel but critical elements in establishing and maintaining hallmarks of human cancer, timely and insightful understanding of their molecular properties and functional mechanisms would pave the road for clinical diagnosis, prognosis, and effective targeting in the global landscape of precision medicine. Further, we address the potential of EVs as promising biomarkers in cancer clinics and summarize the technical improvements in EV preparation, analysis, and imaging. We highlight the practical issues that should be exercised with caution to guide the development of targeting agents and therapeutic methodologies to minimize cancer resistance driven by EVs, thereby allowing to effectively control the early steps of disease exacerbation.